Viscous material, such as mastic caulking material or sealant, is commonly supplied in a plastics cartridge having a discharge nozzle. The cartridge can be mounted in a dispensing gun. An example of such a dispensing gun is described in GB Patent No. 1555455A, incorporated herein by reference. The gun has a plunger on a push rod slidably mounted in a stock. The cartridge is mounted in a keep before the plunger. The plunger is advanced by means of a gripper plate and trigger assembly to force a piston, inside one end of the cartridge, forward to urge the material from a nozzle at the other end. A brake or locking plate is also mounted on the rod. This prevents the advanced piston from retreating until a releasing force is applied to the locking plate. The trigger and gripper plate are reset at the end of a dispensing stroke, while the rod is held in position by the locking plate, and the dispensing force can then be reapplied.
By maintaining a constant force on the trigger, it is found that it is possible to exert a high degree of control over the rate of discharge of the mastic material. However, while the gun is entirely acceptable as a dispensing tool for viscous materials in most situations, it is sometimes desirable to be able to halt the discharge immediately when the hand releases the trigger.
It is found that the body of the cartridge expands radially under a dispensing force. Similarly, if any pockets of gas are trapped in the cartridge they will compress during discharge of the material and expand once the dispensing force is removed from the trigger. The contracting cartridge and any expanding pockets of gas tend to cause continued dispensing of the material when it is not required unless the locking plate is released manually.
U.S. Pat. No. 4,681,524, incorporated herein by reference, discloses a dispensing gun for the dispensing of viscous material from a cartridge in which the discharge of residual material following an application stroke is addressed by urging a locking plate to an engaging position by means of a spring to one side of the push rod. The opposite end of the locking plate is held loosely in an aperture formed in the stock. As the dispensing force is removed, the locking plate retreats within the confines of the aperture, easing the force on the rod exerted by the cartridge.
Another form of pressure release device in a dispensing gun is known from European Publication No. EP 0448315A, incorporated herein by reference, in which the locking plate is frictionally engaged with the push rod by, for example, an O ring such that it is carried by the rod between a released and an engaging relationship as the rod is respectively advanced and retreated. At the end of a dispensing stroke, the force exerted by the cartridge on the plunger results in a small retreating motion carrying the locking plate to its engaging relationship at which point further retreat of the push rod is stopped as the brake plate eventually engages. The resulting “lost motion” releases the pressure on the cartridge by backing off the rod by a small amount, bringing dispensing of the material to an abrupt halt.